1. Field of the Invention
This invention relates to capacitive sensors for measuring the dielectric constant of fluids and for detecting contaminants.
2. General Background and State of the Art
Various geometries have been proposed heretofore for capacitive sensors for liquids, see, for example, U.S. Pat. Nos. 4,470,300, 5,540,086, 5,824,889, 5,900,810, 5,907,278 and 5,929,754. When used to determine the dielectric constant of oil in internal combustion engines, these known sensors with widely spaced capacitive plates, measure the average dielectric constant of the fluids; and accordingly very fine bubbles or finely dispersed droplets of water in the oil indicate a change in the dielectric constant of the oil, rather than identifying the possibly serious problem represented by the presence of water in the engine motor oil.
In addition, the prior art sensors have generally included entirely separate sensing capacitor structures and electronic circuits, with resultant relatively costly construction.
Accordingly, principal objects of the invention include simplifying capacitive sensing assemblies; and providing a sensor which will readily detect contamination such as fine droplets or bubbles of water in oil.
In accordance with one illustrative embodiment of the invention, a capacitive sensing system includes a small printed circuit board with a sensing capacitor formed at one end of the printed circuit board, with the capacitor being formed by a pair of sets of closely spaced conductive lines or pathways printed on the circuit board. Preferably one set of printed conductive lines are closely spaced from a second set of printed conductive lines, to form the two terminals of the capacitor.
The other end of the printed circuit board may have signal processing circuitry thereon, and the printed conductive lines for the signal processing circuitry may be formed concurrently with the conductive lines forming the sensing capacitor.
The printed circuit board may be mounted in a housing, with mounting arrangements for securing the housing for extending into an enclosure, such as an oil pan, containing the liquid to be sensed. The capacitor end of the printed circuit board is exposed to the liquid, and a seal or seals are provided to isolate the signal processing circuitry from the liquid.
The two sets of conductive lines forming the sensing capacitor are preferably closely spaced from one another, preferably in the order of 0.01 or 0.02 inch apart, within the range of about 0.005 inch to about 0.100 inch apart.
In the case of a sensor for determining oil deterioration in an internal combustion engine, and possible water contamination, it is noted that oil has a dielectric constant of about 1.6 to 3.2, depending on brand and age, while the dielectric constant of water is about 87.5 and that of coolant (ethylene glycol) is about 37.0. With the sensing capacitor configurations as shown in the patents cited above, having fairly wide spacing of the capacitor plates, and with water contamination, when it occurs, having very finely divided water droplets or bubbles in the oil, the bulk or average dielectric constant is measured. With oil deterioration being accompanied by an increase in dielectric constant, and with water and coolant having dielectric constants very much greater than oil, a false indication of early oil deterioration may be indicated. In addition, the dangerous condition of water in the oil, possibly indicative of a leaky head gasket or other serious problem, may go undetected.
However, with a sensor of the present invention, the presence of a droplet of water in close proximity to the lines representing the two sides of the sensing capacitor will cause a sharp change in the sensor output, thereby clearly indicating a possible serious engine problem.
Returning now to the sensor assembly, the sensing capacitor conductors, and the conductors for the signal processing circuitry may be formed concurrently, by silk screening a conductive paste onto a small ceramic circuit board, and firing the unit to permanently form and bond the conductive lines or interconnections in place. A thin insulating coating of glass may then be applied over the conductive lines forming the sensing capacitor.
In accordance with a broad aspect of the invention, a capacitive sensor may be formed of a pair of sets of extended conductive lines formed on a circuit board, with the lines being spaced apart by less than 0.10 inch, preferably less than 0.050 inch; and a housing is provided for mounting on a container for fluid, with the capacitive sensor extending into the container and being exposed to the fluid.
Supplemental aspects of the preferred embodiments involve the application of a thin insulating coating, preferably formed of glass, over the printed capacitor, and the use of a fired conductive paste to form the conductive lines on the circuit board.
Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and the accompanying drawings.